1. Field of the Invention
The present invention relates to a CDMA (Code division multiple access) receiving apparatus and a method of detecting a path.
2. Description of the Related Art
A CDMA receiving apparatus is conventionally known which is composed of a finger section, a searcher section and a RAKE synthesizing section, the searcher section is composed of a correlation unit group, an adder group and a path control section. The path control section searches a reception timing with a high level from correlation values before and after the addition, and the finger section determines the reception timing. The finger section detects a valid path at the determined reception timing, and the RAKE synthesizing section RAKE-synthesizes the detected paths.
The path control section in the conventional CDMA receiving apparatus will be described below with reference to FIG. 1.
FIG. 1 is a block diagram showing the structure of the path control section of the conventional CDMA receiving apparatus. As shown in FIG. 1, the conventional path control section 23 is composed of a peak detecting section 31, a threshold value processing section 32, a memory section 33 and a protection processing section 34. Also, the above-mentioned threshold value processing section 32 is composed of a reference threshold value calculating section 322 and a determining section 323. The above-mentioned memory section 33 is composed of a threshold value memory section 331 and a protection path memory section 332.
In the above-mentioned threshold value processing section 32, the above-mentioned reference threshold value calculating section 322 reads a maximum peak level threshold value i and a noise level threshold value j from the above-mentioned threshold value memory section 331. Then, the reference threshold value calculating section 322 calculates a peak level reference threshold value k ((a peak level reference threshold value k)=(the maximum peak level)−(the maximum peak level threshold value i)) from the above-mentioned maximum peak level threshold value i and the maximum peak level which is sent from the above-mentioned peak detecting section 31 (not to shown). Also, the reference threshold value calculating section 322 calculates the noise level reference threshold value l ((noise level reference threshold value l)=(a noise level g)+(a noise level threshold value j)) from the noise level g which is sent from peak detecting section 31 and the above-mentioned noise level threshold value j. Moreover, the above-mentioned reference threshold value processing section 322 outputs the above-mentioned calculated peak level reference threshold value k and the above-mentioned noise level reference threshold value l to the above-mentioned determining section 323.
In the above-mentioned threshold value processing section 32, the above-mentioned determining section 323 carries out threshold value processing to select a path higher than the above-mentioned peak level reference threshold value k and the above-mentioned noise level reference threshold value l from the peak levels f which are sent from the peak detecting section 31. Then, a reception timing of the selected path is set to a search peak timing m. Also, the peak level of the selected path is set to the search peak level n. The above search peak timing m, the above-mentioned peak level, and the search peak level n are outputted to the above protection processing section 34.
The above protection processing section 34 reads the protection path timing p and protection path state q as a result of the protection processing in the previous cycle from the above protection path memory section 332. Then, the protection processing section 34 carries the protection process using the search peak timing m as a reception timing of the path which is found out in a current cycle and determines a valid path. Then, the protection processing section 34 outputs the reception timing of each path determined to be a valid path as a search path timing b to the finger section 11. Also, the protection processing section 34 writes a protection path timing p and a protection path state q as a result of current cycle in the protection path memory section 332.
When the reception timing of the path which has been found out in the processing in the previous cycle is not found out in the processing in the current cycle, it is not determined in the above protection processing that the concerned path is an invalid path, but it is determined that the concerned path is an invalid path when his status continues for a predetermined number of times (front protection processing). In the same way, the path which is first found out in the current cycle is not determined to be a valid path, but the path is determined to be a valid path when the path is found out at the reception timing for the predetermined number of times (back protection processing). This predetermined number of times is possible to set using a parameter. The protection processes is carried out in such a manner that the allocation of the valid path does not change frequently even if the reception level change due to fading and so on and the reception timing changes little.
Next, a specific example of the conventional threshold value process will be described with reference to FIGS. 2A and 2B.
FIGS. 2A and 2B are graphs showing a specific example of the conventional threshold value process.
In the conventional threshold value process, the path equal to or lower than the peak level reference threshold value k is not used due to a peak level reference threshold value k in the region where the propagation environment is good, as shown in 2A, even if a peak level is equal to or higher than the noise level reference threshold value l. The RAKE synthesis is carried out using paths equal to or higher than the peak level reference threshold value k. Also, the path equal to or lower than the noise level reference threshold value l is not used due to the noise level reference threshold value l in the region where the propagation environment is bad, as shown in FIG. 2B, even if a peak level is equal to or higher than the peak level reference threshold value k. The RAKE synthesis is carried out using paths equal to or higher than the noise level reference threshold value 1.
However, there are the following problems in the above-mentioned conventional technique. FIG. 3 is a graph showing a conventional threshold value processing example in the propagation environment in which there is a stronger path so as to be error fee.
The first problem is in that a path unstable near a noise level is used for the RAKE synthesis depending on the value of the maximum peak level threshold value in the propagation environment in which there is so a stronger path as to be error free, so that the reception characteristic is deteriorated, as shown in FIG. 3. The reason is in that means for carrying out the optimal threshold value processing is not provided in case of being the propagation environment in which there has so a stronger path as to be error free in the conventional method of detecting path timings.
FIG. 4 is a graph showing the conventional threshold value processing example in the propagation environment near a sensitivity point.
The second problem is in that all paths corresponding to the peaks which are found out in the current cycle are handled as invalid paths in the threshold value processing depending on the value of a noise level threshold value j, in the propagation environment near the sensitivity point and the path to be used for RAKE synthesis can not be detected so that the reception characteristic is deteriorated, as shown in FIG. 4. The reason is in that means for carrying out the optimal threshold value processing is not provided in case of the propagation environment near the sensitivity point in the conventional method of detecting path timings.
In conjunction with the above description, a spectrum spreading communication apparatus is disclosed in Japanese Laid Open Patent application (JP-A-Heisei 10-164011). In this reference, the spectrum spreading communication apparatus is composed of a plurality of demodulation correlation units for dispreading a reception signal which is subjected to spectrum spreading and for demodulating. A plurality of tracking correlation units are for synchronization tracking of the demodulation correlation units. A search correlation unit searches the phase of demodulation despreading code. A RAKE synthesizing unit synthesizes matches the phases of the outputs of the plurality of demodulation correlation units and carries out a weighting operation. A search processing section sorts the correlation values outputted in order from the search correlation units in a larger order and gives candidates of the phase of the demodulation despreading code to the tracking correlation units. A demodulation path selecting section is provided to be composed of a section for comparing the plurality of peaks outputs from the tracking correlation units with each other. A selecting section selects phases of the above-mentioned peak outputs in order from the maximum peak. A giving section gives the selected phases to the plurality of demodulation correlation units as the phases of the demodulation despreading code.
Also, a spectrum spreading communication apparatus is disclosed in Japanese Laid Open Patent application (JP-A-Heisei 11-4212). In this reference, signals (f1-1 to f1-4) of a plurality of narrowband are extracted from a signal (f1) in a frequency band used for spectrum spreading communication using a plurality of band path filters (7a to 7d), respectively. The level of each of these extracted signals is compared with a predetermined threshold value. It is determined that a reception wave exist when each of all the signals is equal to or higher than a threshold value.
A Cellar system, a mobile terminal, a base station unit and a method of detecting an optimal path are disclosed in Japanese Laid Open Patent application (JP-A-Heisei 11-251962). In this reference, a cellar system using a code division multiple access (CDMA) system is composed of a plurality of finger circuits and a search engine section. The search engine section is composed of a reception level measuring section which detects a reception level of the reception signal and compares the reception level with a predetermined threshold value. A plurality of despreading sections multiply the reception signal and spreading codes. An internal memory stores correlation signals from the plurality of despreading sections. A reception path timing generating section detects a reception path from outputs of the internal memory and generates path timing. It is determined whether or not the correlation signals of the internal memory should be outputted to the reception path timing generating section, in accordance with the comparing and determining result of the reception level measuring section.
Also, a reception timing detection circuit of a CDMA receiving apparatus is disclosed in Japanese Patent No. 2,751,959. In this reference, the reception timing detection circuit of the CDMA receiving apparatus is used for a mobile communication system using a direct spreading code division multiple access (DS-CDMA) system. The reception timing detection circuit is composed of a series correlation unit which calculates correlation signals between a reception signal and a known signal sequence for every predetermined period within a predetermined time interval, and outputs the correlation signals indicating the correlations. An interpolation filter samples the correlation signal again at a frequency which is higher than a sampling frequency and outputs a sampled correlation signal. A power calculating section calculates the power of the sampled correlation signal and outputs the calculated correlation signal powers. An averaging section averages the calculated correlation signal powers over a plurality of periods and outputs an average correlation signal power. A peak detecting section detects a peak of the average correlation signal power, and determines timing when the peak is detected as a reception timing of the CDMA receiving apparatus.
A spectrum spreading communication receiver is disclosed in Japanese Patent No. 2,853,705. In this reference, the spectrum spreading communication receiver is composed of a spreading code generating section for generating a spreading code, and a demodulating section for demodulating a received signal. A demodulation signal is outputted from the demodulating section as a composite data. A searcher section inputs the demodulation signal from the demodulating section and the spreading code from the spreading code generating section and finds a plurality of search paths having correlation peaks which are apart from each other by one or more chips in a search region based on the demodulation signal and the spreading code. A tracking section tracks a plurality of tracking paths which are apart from each other by one or more chips based on correlations between the demodulation signal and the spreading code, and finds correlation levels between the tracking paths. A path capturing and holding section compares the search path from the searcher section and a tracking path from the tracking section, carries out back protection in case of coincidence detection of the paths and carries out front protection in case of extinction of the paths. The path capturing and holding section classifies the path holding state of the tracking path into a complete step out state, a back protection state, a complete protection state, and a front protection state, and holds a plurality of paths. A correlation demodulation path selecting section selects and output a path to be demodulated based on the path state from the path capturing and holding section and the correlation level from the tracking section. A RAKE section detects the demodulation path indicated from the correlation demodulation path selecting section based on the correlation between the demodulation signal from the demodulating section and the spreading code from the spreading code generating section and carries out RAKE synthesis to output as the demodulation data. A demodulating section decodes the demodulation data from the RAKE section and outputs decoding data.